Transcript Work and Energy Notes

Bell Ringer
What do you think of
when you hear the word
energy?
(List at least three items
in your Bell Ringer)
Bell Ringer 10/25
What is another
term for the ability
to do work?
Energy
Energy: The ability of an object to
do work

Units: Joules (J)
Types of energy include:
Mechanical: Energy of movement and
position
 Chemical: Energy stored in chemical
bonds of molecules

Energy
Thermal: “Heat energy” stored in
materials at a certain temperature
 Nuclear: Energy produced from the
splitting of atoms
 Radiant Energy: Energy traveling the
form of electromagnetic waves
 Electric Energy: Energy traveling as
the flow of charged particles
(i.e. electrons)

Work
Work is done when a task produces
a change in energy
Factors affecting work done:
The application of a force
 The movement of the object by that
force over a distance

Bell Ringer
How much work is
required to lift a 2kg
object 2m high?
Work
 Therefore:
Work = Force x Distance
W = Fd
 Units: Joule (J)

1 J = 1 N.m
 Note that work requires a distance
Bell Ringer 3/31
What is another term for the
ability to do work?
You push a stationary wall with
a force of 1000N. How much
work was done to the wall?
Bell Ringer
Power
How much work is performed
over a period of time
Therefore:
Power = Work / Time
P = W/t
Units: Watts (W) where 1 W = 1 J/s
Thought Question
How many
horses are in
one
horsepower?
Power
Power can also be converted to
units of horsepower (hp)

Note: 1 hp  750 W
coffee maker
blender
lawn mower
Corvette
0.75 hp
1.5 hp
5-6 hp
400 hp
Bell Ringer
If Superman, at 90kg, jumps a
40m building in a single bound,
how much does Superman
perform?
If this occurs in 3s, what is his
power output?
Energy
The amount of work done by an
object does not depend on the path
taken
 Work depends only on the object’s
starting and ending points

As work is done on an object, the
object itself gains the opportunity to
do work
Energy
For example:
A bowstring drawn back on a bow
 Winding an alarm clock
 Raising the arm on a pile driver

All of these objects now have
the ability to do work
Mechanical Energy
Mechanical Energy: Energy
of movement and position

There are two major types of
mechanical energy:
Potential
Energy: Energy of
position
Kinetic Energy: Energy of motion
Potential Energy
Gravitational Potential Energy:
The potential due to elevated
positions
P.E. = mass x gravity x height

P.E. = mgh
Recall: weight = mass x gravity

Therefore: P.E. = weight x height
Potential Energy
Kinetic Energy
Objects in motion are capable of
doing work
.
.
2
KE = ½ mass velocity
KE =
2
½mv
Kinetic Energy
Note that the velocity of the object
is squared when determining KE
 If the velocity of the object is
doubled, the KE is quadrupled
Energy Conservation
Energy is constantly
transforming, but never
“disappears”
Law of Conservation of Energy:
Energy cannot be created or
destroyed, only changed from
one form to another.
Energy Conservation
Potential and kinetic energy are
constantly transforming back and
forth

Most of the time during this
transformation, some energy is turned
to heat and transferred out of the system
Energy Conservation
Bell Ringer
 Jill has a velocity of 5m/s. If she has a mass
of 60kg, what is her kinetic energy?
 If Bob, at 70kg, is standing on top of a 13m
high hill. What is his potential energy?
Work-Energy Theorem
The change in gravitational
potential energy of an object is
equal to the amount of work
needed to change its height
Therefore:
Work = DPE
Fd = mgh
Work-Energy Theorem
The KE of a moving object is
equal to the work the object is
capable of doing while being
brought to rest
Therefore:
W = DKE or Fd =
2
½mv
Work-Energy Theorem
Putting these two ideas together gives
us the general Work-Energy
Theorem:
If no change in energy occurs,
then no work is done. Therefore,
whenever work is done, there is
a change in energy.
Bell Ringer
List and give an
example of the 6
types of simple
machines.
Simple Machines
Machine: A device used to multiply
forces or to change the directions of
forces
There are six types of simple
machines:

Pulley: Grooved wheels which assist in
raising, lowering, or moving an object
Simple Machines
Lever: A stiff bar which pivots on a
support to assist in lifting or moving an
object
 Wedge: An object consisting of a
slanting side ending in a sharp edge
which separates or cuts materials apart
 Wheel and Axle: A wheel with a rod
through its center which lifts or moves
objects

Simple Machines
Inclined Plane: A slanting surface
connecting a lower level to a higher
level
 Screw: An inclined plane wrapped
around a rod which holds objects
together or lifts materials

Bell Ringer
What is an example
of a 100% efficient
machine?
Mechanical Advantage
 Mechanical Advantage: A machine’s ratio
of output force to input force
Mechanical Advantage = Output Force
Input Force

i.e. A machine which outputs 80 N for every
10 N you put in has a mechanical advantage
of 8.

Note that the load will move only 1/8 of the
input distance
Efficiency
Efficiency: A machine’s ratio of
useful work output to total work input
Efficiency = Useful Work Output
Total Work Input
Efficiency is expressed as a percent

i.e.) An efficiency result of 0.25 means
25% efficiency
Efficiency
Ideal machines have 100% efficiency

This means that all of the energy put
into the machine exits as useful energy
All other machines will ALWAYS
have an efficiency of less than 100%
 A machine cannot output more
work than is put into it
Pulleys
Single Pulley:

Changes the direction of a force
exerted by a rope or cable
System of pulleys:

Multiplies input forces, creating
large output forces
Pulleys
• Each supporting strand of rope holds an
equal fraction of the weight
• Tension in this cable is the same
throughout its entire length
• Input force = tension in each
supporting segment of the cable
• Mechanical advantage = number of
supporting strands
Pulleys
Input force = 30 N
30 N
Pulleys
Input force = 15 N
30 N
Bell Ringer
 How many supporting
strands are there ?
 What is the
Mechanical advantage
here equal to?
 What is the input
force required to lift
the 200kg object?
More Practice
 What is the minimum effort that
must be applied to lift the load?
 For every 2 meters the rope is
pulled through what height does
the load rise off the ground?
 What is the mechanical
advantage?
LEVERS
Levers
A simple machine made of a bar
which turns about a fixed point

Fulcrum: The pivot point of a lever
Change the direction of or
multiply input forces
Three Types of Levers
Type 1 Lever: Fulcrum lies between
the input force and the load
 i.e.) A seesaw
Type 2 Lever: The load lies
between the fulcrum and the input
force
 i.e.) A pry bar
Three Types of Levers
Type 3 Lever: The input force lies
between the fulcrum and the load
 i.e.) Your forearm pivoting about
your elbow
Lever Lab
Levers
If friction is small enough to neglect:
Work Input = Work Output
or
(Fd)input = (Fd)output
Therefore: A small input force over a
large distance will output a large
force over a small distance
Levers
Levers
Wedge

Wedge: An object consisting of a
slanting side ending in a sharp edge
which separates or cuts materials
apart
 i.e.
knife
Wheel and Axel

Wheel and Axle: A wheel with a rod
through its center which lifts or
moves objects

ie: Cart
Inclined Plane

Inclined Plane: A slanting surface
connecting a lower level to a higher
level
 i.e.
Accessibility ramp
Screw
Screw: An inclined plane wrapped
around a rod which holds objects
together or lifts materials
Compound Machine
 Compound machines use two or simple
machines to complete a task
 Examples?

Rube Goldberg Device
Bell Ringer
How much energy is
transferred in lifting a 5 kg
mass 3m?
What is the work energy
theorem?